This invention relates to the preparation of 3,3-dimethyl butanal, and more particularly to improved processes for preparing 3,3-dimethylbutanal and precursors therefor.
Nofre et al. U.S. Pat. No. 5,480,668 describes artificial sweetening agents comprising N-substituted derivatives of aspartame. A preferred example as described by Nofre is N-[N-(3,3-dimethybutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester. As the reference further describes, this product may be produced by reaction of 3,3-dimethylbutyraldehyde (3,3-dimethylbutanal) with aspartame and a reducing agent such as sodium cyanoborohydride in a solvent medium such as methanol.
Nofre U.S. Pat. No. 5,510,508 and Prakash U.S. Pat. No. 5,728,862 describe preparation of N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine, 1-methyl ester by a reductive alkylation reaction comprising catalytic hydrogenation of the Schiff's based produced by condensation of 3,3-dimethylbutanal and aspartame.
To facilitate the manufacture of N-[N-(3,3-dimethybutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester, there has been a need in the art for an improved and economical method for preparation of the 3,3-dimethylbutanal intermediate. Currently, 3,3-dimethylbutanal is available on the market only in limited quantities at prices that are very expensive. Previously available manufacturing processes have generally failed to provide satisfactory yields, or to produce an aldehyde intermediate of adequate purity, substantially free of by-products, such as t-butylacetic acid. Recently, improved methods have been developed, but a need has remained for a more satisfactory process for the commercial manufacture of 3,3-dimethylbutanal.
Prakash et al. U.S. Pat. No. 5,856,584 describes a process for the preparation of 3,3-dimethylbutanal by oxidation of 3,3-dimethylbutanol. Oxidizing components used in the process include an oxidizing metal oxide or 2,2,6,6-tetramethyl-1-piperidinyloxy, free radical, and an oxidizing agent such as sodium hypochlorite. Oxidation by a metal oxide may effected by contacting 3,3-dimethylbutanol in a vapor phase comprising an inert carrier gas with an oxidizing metal oxide. Oxidation by 2,2,6,6-tetramethyl-1-piperidinyloxy, free radical, and sodium hypochlorite may be conducted in a solvent system.
Slaugh U.S. Pat. No. 4,891,446 describes a process for catalytic dehydrogenation of saturated primary alcohols having a carbon number ranging from 5 to 20, preferably 7 to 18. The alcohol is passed over a fixed bed of brass particles in a vertical column or horizontal tubular reactor. Although hydrogen is a product of the reaction, additional hydrogen is introduced into the reactor in order to obtain good catalyst life or stability. Specific working examples are given for dehydrogenation of C9, C13, and C15 alcohols.
Gulkova et al. “Dehydrogenation of Substituted Alcohols to Aldehydes on Zinc Oxide-Chromium Oxide Catalysts,” Collect. Czech. Chem. Commun., Vol. 57, pp. 2215–2226 (1992) reports the exploration of sixteen primary alcohols for the possibility of obtaining the corresponding aldehydes by dehydrogenation on solid catalysts. Among the substrates tested was 3,3-dimethyl-butanol. The reference reports certain rate constant information for the dehydrogenation of a 3,3-dimethylbutanol substrate, but does not include description of the particular conditions under which this substrate was converted to 3,3-dimethylbutanal.
Banthorpe et al., “Mechanism of Elimination Reactions. Part XX. The Inessentiality of Steric Strain in Bimolecular Olefin Elimination,” J. C. S., 1960, pp. 4084–4087 describes the dehydrogenation of 3,3-dimethylbutanol to 3,3-dimethylbutanal in which 3,3-dimethylbutanol was boiled into a vertical tube containing pumice supported copper chromite catalyst and surmounted by a reflux condenser. The catalyst became reduced after 40 minutes reaction and was regenerated by exposure to an air stream at 320° C. for 2 hours.
Where 3,3-dimethylbutanal is derived from 3,3-dimethylbutanol, the provision of a satisfactory commercial method for the preparation of the aldehyde also requires the selection and/or development of an economically effective method for the preparation of the alcohol.
Hoffman et al. U.S. Pat. No. 3,754,052 describes reaction of isobutylene, ethylene and sulfuric acid in the presence of isobutane to produce the sulfate ester of 3,3-dimethylbutanol in isobutane. Unreacted ethylene is removed and the sulfate ester is alkylated with isobutane at ≧25° C. to produce 2,3-dimethylbutane.
Wiese U.S. Pat. No. 2,660,602 describes a process for the preparation of branched primary sulfate esters by reaction of ethylene, an olefin co-reactant and sulfuric acid, particularly including the preparation of 3,3-dimethylbutyl hydrogen sulfate where the olefin co-reactant is isobutylene. The reaction is carried out by simultaneously contacting strong sulfuric acid with ethylene and the co-reactant, preferably in the cold. A high ethylene to co-reactant ratio is maintained. A hydrocarbon diluent is preferably present in the reaction zone, particularly when employing low molecular weight co-reactants, i.e., less than C12. The reference disclosed hydrolysis of 3,3-dimethylbutyl monohydrogen sulfate ester to 3,3-dimethylbutanol, and further suggests the preparation of the acetate of the alcohol, which is said to be useful as a lacquer solvent. Wiese et al. also suggest preparation of di-octyl phthalate ester plasticizers by esterification of phthalic anhydride with branched chain alcohol.
Reactions have been described in which carboxylic acids and esters are reduced to alcohol by reaction with strong reducing agents commonly lithium aluminum hydride. Such reactions must be handled with caution due to the high reactivity of the reducing agents. Journal of Organic Chemistry, Vol. 46 (1981), pp. 2579–2581 discloses that carboxylic acid amides can be reduced to the corresponding amines by a combination of sodium borohydride and methane sulfonic acid and dimethylsulfoxide (DMSO). The same paper discloses that acetic acid and phenyl acetic acid can be reduced to corresponding alcohols under similar conditions. However, the paper contains no suggestion of the reduction of other acids to the corresponding alcohols. Sodium borohydride is a widely used reducing agent and relatively safe to work with but has not been considered generally suitable for reducing the carboxylate group due to its mild reducing capacity.
Journal of the American Chemical Society, Vol. 73 (1951), p. 555 discloses hydrolysis of 1-chloro-3,3-dimethylbutane with potassium carbonate in a closed system to produce 3,3-dimethylbutanol in 65% yield. Since carbon dioxide is generated in the reaction, the procedure requires operation at high pressure to avoid stripping out the aqueous phase.